ada: Fix bogus error about null exclusion for designated type with multiple views

It comes from the new legality check retroactively introduced by AI12-0289,
but the implementation is fooled by the presence of both an incomplete and
a partial view for a tagged type.

gcc/ada/ChangeLog:

	PR ada/111433
	* sem_util.ads (Incomplete_Or_Partial_View): Add Partial_Only formal
	parameter.
	* sem_util.adb (Incomplete_Or_Partial_View): Likewise.  Do not look
	for an incomplete view if it is set to True.
	* sem_ch6.adb (Check_Conformance.Null_Exclusions_Match): Pass True
	for Partial_Only in the call to Incomplete_Or_Partial_View.